We are investigating the light regulation of the levels of translatable mRNAs coding for two specific polypeptides. These two polypeptides are the small subunit of ribulose 1, 5-bisphosphate arbosylase, the major soluble chloroplast protein, and the chlorophyll a/b binding protein, the major protein of thylakoid membranes. We are cloning cDNA copies of the mRNAs in order to develop hybridization probes for the RNA sequences for these two polypeptides. The use of these probes, along with the in vitro translation assay and immunoprecipitation of these polypeptides, will allow us to examine in detail the possible regulation by light of the transcription, processing, and/or translation of these specific mRNAs. We have now shown that phytochrome is involved in one of the regulatory steps. We shall also characterize the receptor(s) for the light regulated step(s).